The present invention relates to differential screw actuators; and more particularly, but not exclusively, the invention relates to improvements in ball-screw-actuating and operating mechanisms for a mirror mount, or the like, such as described in my U.S. Pat. No. 4,060,314, whose content is incorporated by reference. Broadly speaking, the invention relates to precision actuation, to obtain, provide, and retain small, very well defined positions and displacements.
The patent mentioned above discloses a structure for tipping and tilting a laser mirror. The terms TIP and TILT are used to differentiate among two different pivot motions about two intersecting axes in the plane of the mirror. The mirror is mounted directly to a backup plate which, in turn, is cantilevered to a support by means of flexure elements. Mirror and backup plate are tilted and tipped in relation to the support (while continued to be held by the flexure elements) by means of two so-called differential ball screw actuators Each actuator includes screws and two sets of bearing balls encapsulated within an enclosed continuous helical track at both ends. The helical track at one end has a pitch angle different from the helical track at the other end to provide differential movement between the two ends as the screw is rotated either clockwise or counter-clockwise. The ball bearings within the respective, enclosed continuous track are in compressional contact with the walls formed by the respective, enclosed continuous track, the compressional force being supplied by the flexure elements between the backup plate and the fixed support. The combined deflection of both ball and contact surface of the walls averages out any error introduced between the multiplicity of balls which provides an adjustment precision of the backup plate with respect to the fixed support of about 0.5 in..times.10.sup.-6 in. or better. Positioning of the backup plate is achieved by driving each of the two differential screw actuators with stepping motors with back-lash compensation, making it possible to accurately position the mirror. The differential ball screw in combination with gear reduction provides, for example, less than one microinch of mirror movement per step of actuator motion. The actuators are locked in place by the detent action of the stepping motors with or without the fields energized, thus eliminating mirror misalignment which often occurs during locking of conventional mounts when set screws or the like are manually tightened. The ability of the device to retain a particular adjusted position and the overall stability are significant advantages of this device.
It can readily be seen that the precision of actuation by such a ball screw actuator depends to some extent upon the actual difference in pitch of the front and rear parts of the screw. The smaller that difference, the greater the precision. This, however, entails a longer screw to provide the required actuated stroke. One needs, therefore, deep envelopes receiving long screws. Since such a screw may have to be several feet long, correspondingly long envelopes may simply not be available. Moreover, long screws and deep envelopes are expensive because, in each instance, precision must be maintained for great lengths.